Protective device.



S. DUSHMAN.

PROTECTIVE DEVICE. APPLICATION FILED MAY 20, 1915.

1, 172,96 1. Patented Feb. 22, 1916.

Fig. I.

In ventor: Saul Dushman,

VOLTAGE His fittorney.

UNITED STATES PATENT; OFFICE.

SAUL DUSHMAN, 0F SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRICCOMPANY, A CORPORATION OF NEW YORK.

PROTECTIVE DEVICE.

Application am: m 20, 1915. Serial No. 29,453.

To all whom it may concern: 7

Be it known that I, SAUL DUs naaN, a subject of the King of Great Brtain, residing at Schenectady in the county of Schenectady, State of NewYork, have nvented certain new and useful Improvements in ProtectiveDevices, of which the following is a specification.

My invention relates to the protection against overload of devicespassing a sub-- stantially constant current with a variable impressedvoltage, such,-for example, as ncandescent cathode devices, wh1ch whenoperating in series with a load device are liable to injury ordestruction should the load device or devices be short-circulted or inany other way consume less voltage and 7 thereby cause an excessivevoltageacross the constant current device.

Although my invention is of general application it may be betterunderstood by a consideration of the electric characteristics ofincandescent cathode electron discharge devices. When such a device isso highly exhausted that its current is a pure electron discharge,relatively large amounts of energy may be transmitted. Over a glvenrange, varying with the geometric relation in space of the parts of. thedischarge device, the distance between the electrodes and other factors,the current varies with the 3/2 power of the voltage and then at highervoltages the current becomes substantially constant with a variableimpressed voltage.

When the voltage impressed is greater than i this critical value, theresistance of the external load may be varied without afiectmg thecurrent. The latter then depends only upon the temperature of the sourceof electron discharge. During normal operat on only a small part of thevoltage of the source is consumed in the tube, the rest of the voltagebeing consumed in the load.

Should the load be short-circuited all of the voltage, and hence all ofthe energy of the system, would be consumed in the tube, and unless somesafety device were provided the tube would be destroyed. Ordinaryprotective devices would not protect the tube under these circumstances.In the first place, since the current remains constant, an excesscurrent device would not operate. Secondly, an excess voltage devicecould not be used since the full voltage that occurs across the tubewhen current is flowing on Specification of Letters Patent.

short-circuit exists in normal operation on every other half cycle whilecurrent has ceased to flow because of the unidirectional conductivity ofthe tube. This high voltage that exlsts normally while current is notflowing does no harm to the tube since it represents no energy, but itmakes impossible the utilization of the usual excessvoltage devices toprotect'the tube.

In accordance wlth my invention I utilize the increase in voltage dropacross the device to be protected to effect-an increase of currentpassed by an auxiliary electron discharge apparatus when the current isof a. polarity to cause a rise of energy in the main device. Th lsauxiliary device is-constructed to requlre a high voltage to pass anappreciable current, and in which the current under normal operatingconditions low the saturation value ther increase in voltage thercurrent. This increase of current in the auxiliary device actuates anyknown form of current relay to deenergize the main dlscharge device, andalso deenergizes the auxiliary discharge device.

My invention will be better understood by reference to the followingdescription taken in connection with the accompanying drawing'in whichFigure 1. illustrates grammatically load circuit supply by half waverectifiers, and Fig. 2 illustrates by curves the relation of the voltageand the current in the main and the auxiliary devices.

Referring to the drawing the main electron discharge device 1, which inthis case is shown as supplying the load 2 with rectified current isconnected to the secondary of a transformer 3, by the conductors 4, 5."The discharge device contains an incandescent cathode 6, and an anode7. The electrodes are so completely freed from gas and the space in theenvelop evacuated to a pressure so low that transmission of energyoccurs by pure electron conduction substantially without gas ionization.With this 'end in view the bulb is first evacuated by approved lampexhaust methods, the last portlons of gas being removed by the Gaedemolecular pump or any other means capable of removing vapors of oil andwater aswell as gases. When the evacuation has proceeded to such a highdegree that an electron dis;harge of is considerably beat which anyfurproduces no furmy invention diaconsiderable magnitude may be through;

the device w thout destructive positive ion1-;- --zation,

the cathode is heated to incandescence and a high potential, forexample, 5000'volts and higher, impressed upon the electrodes. Thebombardment of the anode by the electron discharge liberates therefromionizable gas. As soon as a blue low is observed the discharge isdiscontinued and umping continued until a high vacuum has con restoredwhen the process is repeated. When the electrodes have been thussuiticiently freed from gas to operate at high voltages without givingolf gases and the space within the tube has been exhausted to a vacuumof about .05 microns (0.00005 m. m. of mercury) or lower, the tube issealed oil from the pump. The cathode 6 is heated to incandescence by abattery 8 connected to the cathodeby the conductors 9, 10, having incircuit a switch 11, the ob ect of which will be presently explained.The electrodes of the device are spaced closely together so as todecrease the and consequent energy loss. The eflectof this is to lowerthe voltage limit above wh ch the device operates with a constantcurrent with a-variable impressed voltage.

As explained by Dr. Irving Langmuir in the PitysicaZReoiew for December1913, the electrostatic and magnetic influence of each electron in agiven space on every other electron in the space between the electrodesof the device is known as the space charge efi'ect. Owing to this spacecharge efi'ect the electron current varies over a given range with theimpressed voltage, providing the temperature of the filament is so highthat there is no limitation owing to the inability of the cathodetosupply electrons. At voltages. above this range the current remainssubstantiallyv constant. As noted above, the

range of space charge voltage depends on the geometrical relations ofthe parts of the device. The device 1 is constructed to operate with alow space charge voltage to redues to a minimum the loss of energy inthe tube. For example, as shown in Fig. 2, the electrical characteristicof the device may be represented by the curvev A, the ordinates beingcurrent value in milliamperes, and the abscissae voltage values. In thisparticular .device with the filament at a given temperature the currentincreases with the 3/2 power ofthevoltage up to 100 volts when all-theelectrons which the filament is capable of emittingat that temperatureare utilized; at voltages exceeding 100 volts the current remainsconstant.

Connected across the terminalsof therectifier 1 is a second rectifyingdevice 12, also having an electron-emitting cathode 13 and an anode 14:.In'this device the electrodes are so arranged, for example, byspacingthem more widely apart, that-the range over I voltage drop W rn .rvhichthe current varies with the volta e is vary greatly increased,accompanied by. a variable current including the range of V0 tags theworkingrange of voltage drop acrossthe terminals of the device 1.

Again referring to Fig. 2, the electrical characteristics of the device12 may be represented by the curve B. The current increases with the 3/2power of the volta e multipled by a constant depending on t e electronemission up to nearly 600 volts, and then becomes substantially constantat 80 milliamperes, the electron emission being somewhat less than inthe device ,1, for example, because of a lower filament temperature.

The battery 15 heating the cathode 13 has included in its circuitconductors 16, 17 and a switch 18 connected to the same shaft 19 as theswitch 11. In series with the device 12 is the winding of anelectromagnet 20 having its core connected to the shaft 19. Under normaloperating conditions the space charge voltage across device 1, andconsequently across device 12, is so low that the current passingthrough the device 12 is so small that it is insuflicient to operate themagnet 20. On the other hand, when a short circuit or sudden decrease inresistboth the devices 1 and 12 becomes so high that the current throughdevice 12 becomes sufficiently great to cause the magnet 20 to raise itscore, opening the switches 11 and 18, and closing the switch 23. In thegiven case this tripping current might be set at 40 milliamperes. Theeffect of this isto deenergize the heating circuits of the cathodes ofboth the devices 1 and 12 by opening the switches 11 and 18 and toconnect the winding of the magnet 20 across the battery 15 by theconductors 16, 21, and 22 by the closure of a switch 23. When thedisturbing conditions have been corrected the switches 11 and 13 may beclosed and for example, by opening a switch 24 in the circuit 16.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, 1s:

1. The combination of a source of alternating current, an electrondischarge device, conducting a constant current with a variableimpressed voltage exceeding a predetermined value, connected to saidsource, a load connected in series with said device, an auxiliaryelectron discharge device con ducting a variable current at an impressedvoltage exceeding the predetermined operating voltage of the firstdevice, and means responsive to a predetermined rise of current in thesecond device to deenergize the first discharge device.

2. The combinatlon of a discharge device having an electron-emittingcathode and being evacuated to a pressure so lOW that con-' duction ofenergy occurs substantially Without posit e nization, a load connectedin series With e device, a source of alternating current connected tosupply current to said device and said load at a voltage exceeding thevoltage at which the current of said device becomes substantiallyconstant, a second pure electron-discharge device connected in par llwith the first device constructed to ate with variable current over avolta range exceeding the range of voltage over which the current isvariable in the first den and means responsive to a predetermin rise ofcurrent in said second device to dei s ize both of said devices.

3. The conbination or". an electrical discharge device operating with asubstantially constant current over a variable range of impressedvoltage exceeding a predetermined value, a load in series with saiddevice, rectifying means connected in parallel 'With said device,transmitting current of the same polarity as said discharge device, thecurrent being variable with the voltage nected to said means fordenergizing said discharge device upon a predetermined rise of current1n said rect1f mg means.

4. The combmatron o a rectifier operating with a substantially pureelectron discharge and having electrodes spaced closely to keep theenergy loss due to space charge voltage drop at a low value, a secondrectifier operating with a substantially pure electron discharge havingelectrodes spaced more Widely apart to give a high space charge voltage,and connected -to corresponding electrodes of the first rectifier, asource of electrical energy supplying said rectifiers, a load in seriesWith both said rectifiers and an electroresponsive device operativelyconnected to said second rectifier for denergizing both said rectifiersupon a predetermined rise of current.

In Witness whereof, I have hereunto set my hand this 18th day of May,1915.

' SAUL DUSHMAN.

